acoustics and noise control, and frequency weighting in general—including weighting curves—is described in the online encyclopedia *Wikipedia* (*http://en.wikipedia.org/wiki/Frequency_weighting*). Octave and one-third octave frequency bands (*http://www.diracdelta.co.uk/science/source/o/c/octave/source.html*) are also used for a more complete description of the frequency spectrum of noise (see examples in this report).

The *decibel*, unfortunately for public comprehension, is used in a variety of ways in noise control and other branches of engineering. That it involves a logarithm makes math-averse individuals uncomfortable. The decibel was originally used in the Bell telephone system to describe the attenuation of a mile of “standard cable.” It is also commonly used to describe the gain of an amplifier and the power delivered to an electrical load. The online encyclopedia *Wikipedia* is a good source of information for a basic understanding of the concept (*http://en.wikipedia.org/wiki/Decibel*).

The decibel is firmly entrenched in the language of noise, as in “how many decibels of noise is that?” Noise “thermometers” are frequently published showing the decibel level of noise for various sources. Examples are given in Chapter 1. These levels are almost always measures of noise *immission*.

Fundamentally, the decibel is a unit of level and is defined as 10 *logQ*/*Q*_{ref}, where Q is a quantity related to energy and *Q*_{ref} is a reference quantity. It is the fact that both *Q* and *Q*_{ref} can be different quantities (squared pressure, power, intensity, etc.) that makes general use of the decibel even more confusing to the public. The mean square pressure is the quantity most commonly used to describe noise, and its corresponding reference quantity is (20 micropascals)^{2} or, in terms of Newton per meter squared, (2 · 10^{–5} N/m^{2})^{2}. Given the range of mean square pressures commonly encountered when dealing with noise, the sound pressure level generally ranges from about 0 to 140 dB. The corresponding pressures are only a tiny fraction of atmospheric pressure. Although the A-frequency weighting described above applies to the signal and not to the unit (dB), the A-weighted sound pressure level is often expressed as dB(A) or dBA.

Even with one definition of Q as the mean square pressure, different averaging times lead to different decibel values—which causes further complication. For example, in the evaluation of hazardous noise in the workplace, an 8-hour average is commonly used. For environmental noise outdoors, a day-night average sound level is computed by using A-frequency weighting and averaging the mean square pressure over 24 hours with an increase in the amplification of the measuring system of 10 dB during the nighttime hours.^{1} This quantity is the day-night average sound level, *L*_{dn} (DNL). To add further complication, it is common European practice to use a 5-dB amplification in the measuring system during the evening hours and a 10-dB gain during the nighttime hours. The result is the day-evening-night level, *L*_{den}.

Another important quantity is sound exposure and the corresponding *sound exposure level* in decibels. This measure is useful for assessing the noise produced by single events such as an airplane flyover or vehicle pass-by. Here, the quantity *Q* is the time integral of the squared pressure over the time interval of the event. The reference quantities are 20 micropascals as the reference pressure and 1 second as the reference time.

The decibel is also used in noise control for *sound intensity* and *sound power*, which are common descriptors of noise *emission*. For sound intensity level, the quantity is sound intensity and the reference quantity is 10^{–12}*W*/m^{2}. For sound power level, the quantity is sound power and the reference quantity is 10^{–12}*W*. In the information technology industry, the sound power level is commonly expressed in bels, B (10 dB = 1 B) to avoid confusion between sound pressure level and sound power level. This has not been widely adopted, however. For example, European requirements on outdoor equipment are based on the sound power level in decibels.

Throughout this report, the terms *sound pressure level*, *sound intensity level*, and *sound power level* are used to clarify which level is being discussed. The term *sound level* is sometimes used when sound pressure is implied—such as in day-night average sound level. It is also used in connection with instruments—such as sound-level meter—and when the quantity being discussed could be either pressure, intensity, or power.

Crocker, M.J., ed. 2007. Handbook of Noise and Vibration Control. Hoboken, N.J.: John Wiley and Sons.

Rossing, T., ed. 2007. Springer Handbook on Acoustics. New York: Springer Science+Business Media LLC.

Vér, I.L., and L.L. Beranek, eds. 2006. Noise and Vibration Control Engineering. New York: John Wiley and Sons. See, for example, Beranek, L.L., Chapter 1, Basic Acoustical Quantities: Levels and Decibels.